fix(path): normalize path slashes on Windows #37729

Problem:
On Windows, path separators may become inconsistent for various reasons,
which makes normalization quite painful.

Solution:
Normalize paths to `/` at the entry boundaries and always use it
internally, converting back only in rare cases where `\` is really
needed (e.g. cmd.exe/bat scripts?).

This is the first commit in a series of incremental steps.

Note:
* some funcs won't respect shellslash. e.g. `expand/fnamemodify`
* some funcs still respect shellslash, but will be updated in a follow
  PR. e.g. `ex_pwd/f_chdir/f_getcwd`
* uv's built-in funcs always return `\`. e.g. `uv.cwd/uv.exepath`

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

test/functional/vimscript/buf_functions_spec.lua

@@ -6,7 +6,6 @@ local clear = n.clear
 local fn = n.fn
 local api = n.api
 local command = n.command
-local get_pathsep = n.get_pathsep
 local rmdir = n.rmdir
 local pcall_err = t.pcall_err
 local mkdir = t.mkdir
@@ -81,8 +80,8 @@ describe('bufname() function', function()
   it('returns expected buffer name', function()
     eq('', fn.bufname('%')) -- Buffer has no name yet
     command('file ' .. fname)
-    local wd = vim.uv.cwd()
-    local sep = get_pathsep()
+    local wd = assert(t.fix_slashes(assert(vim.uv.cwd())))
+    local sep = '/'
     local curdirname = fn.fnamemodify(wd, ':t')
     for _, arg in ipairs({ '%', 1, 'X', wd }) do
       eq(fname, fn.bufname(arg))